As is well known, in recent radiation therapy, attention has been centered on advanced stereotactic radiation therapies such as three-dimensional conformal radiation therapy (3D-CRT) and intensity modulated radiation therapy (IMRT), which are intended to deliver radiation such as hard X-rays, electron beams, or accelerated particle beams while appropriately setting the shape and dose level of the radiation to be delivered (see, for example, Non-patent Document 1). In these therapies, various parameters such as irradiation site, coverage, and output are set using, for example, a therapy planning apparatus, before delivering radiation. By this means, the effort of, for example, delivering a high dose of radiation to only a lesion while avoiding organs-at-risk neighboring the lesion, has been made to accomplish precise treatment. Hence, in these radiation therapies, it is important to set the above various parameters to appropriate values. Other required matters are a high level of mechanical precision of an irradiator apparatus itself and a high level of precision in the control of various filters, line width enlargement devices and the like with which the irradiator apparatus is equipped.
Hence, in implementation of the above radiation therapies, it is necessary to verify the settings of various parameter values and the precision by measuring the doses of radiation to be used in the therapies. Particularly, as to the stereoscopic dose distribution of radiation near a lesion to be irradiated with the radiation, much empirical data are required to be obtained.
A thermoluminescent layered product formed by layering a plurality of thermoluminescent plates stereoscopically is well known as a dosimeter for measuring the stereoscopic dose distribution, i.e., three-dimensional dose distribution, of radiation (see, for example, Patent Document 1).
According to Patent Document 1, the thermoluminescent plate constituting the thermoluminescent layered product is in the shape of a flat plate and formed of, as a material, a thermoluminescent substance, more specifically, a thermoluminescent substance comprising lithium tetraborate or the like as a base material and manganese as a luminescent center added to the base material. By delivering radiation to the thermoluminescent layered product comprising the thermoluminescent plate, the three-dimensional dose distribution of the radiation can be obtained.
More specifically, after being irradiated with radiation, the thermoluminescent layered product is divided into the thermoluminescent plates and then each of the thermoluminescent plates is heated. From the thermoluminescent plates, the light intensity distribution of thermoluminescence caused by this heating is acquired. As is well known, there is a certain relation between the light intensity of thermoluminescence and the radiation dose. Hence, from the information on the light intensity distribution thus acquired, the information on the planar exposure dose distribution (hereinafter, also referred to simply as “dose distribution”), i.e., two-dimensional dose distribution, of radiation along a surface irradiated with the radiation can be obtained. The thus obtained information on each dose distribution is reconstructed as information on the dose distribution of the radiation delivered to the original thermoluminescent layered product, whereby a stereoscopic, i.e., three-dimensional dose distribution can be acquired.
Further, according to Patent Document 1, the thermoluminescent plate constituting the thermoluminescent layered product is adjusted to be tissue equivalent to living tissues constituting the human body (e.g., muscle tissue), that is, to be equivalent in effective atomic number to such living tissues.
Hence, in the thermoluminescent layered product described in Patent Document 1, when irradiated with radiation, effects such as photoelectric interaction, Compton effect, and electron pair producing effect are produced at the same level as those in the human body. Thus, when such a thermoluminescent layered product is used as a dosimeter, data on the dose of radiation delivered to the human body can be acquired directly from measured values without making various corrections.
In addition to the manganese-containing lithium tetraborate (Li2B4O7) disclosed in Patent Document 1 referred to above, for example, the thermoluminescent substances described below which can be used as materials for a dosimeter are well known (see, for example, Non-patent Documents 2, 3, and 4).
That is, Non-patent Documents 2 and 3 each disclose that copper-containing lithium triborate (LiB3O5) is a thermoluminescent substance exhibiting thermoluminescence.
Non-patent Document 4 discloses that a thermoluminescent substance can be obtained as a composition produced by thermal reaction of lithium tetraborate, boron oxide, and copper(II) oxide. Non-patent Document 4 also discloses that thermoluminescent substances having various properties are formed according to the mixing ratios of these materials, i.e., lithium tetraborate, boron oxide, and copper(II) oxide by varying the ratios.
To prepare a dosimeter for measuring three-dimensional dose distribution by using the thermoluminescent substance of Non-patent Document 2, 3, or 4 as a material for the dosimeter, first, a resin is used as a binder to mold the thermoluminescent substance into a product in the shape of a flat plate, as disclosed, for example, in Patent Document 2. A plurality of the plates are then layered in the same manner as in the case of the thermoluminescent plate disclosed in Patent Document 1 described above, whereby it is deemed that a thermoluminescent layered product can be prepared.